Low-temperature raman spectra of polycrystalline NH4F and ND4F

Raman spectra of polycrystalline ammonium fluoride and fully deuterated amm onium fluoride were recorded at various temperatures between 80 and 300 K. The spectra were obtained using a cold stage attached to a Raman microscope . At low temperatures the Raman lines were sharper and shifted to higher wa venumbers compared with their room temperature counterparts. However, no in dications of any crystalline phase change were observed at low temperatures . A strong line at 2876 cm(-1) was assigned to the infrared inactive symmet ric NH stretching mode v(1)(A(1)). A very strong line at lower wavenumbers (2818 cm(-1)) was assigned to the antisymmetric stretching mode v(3)(F-2) O bserved deuterium isotope shifts were in agreement with those calculated us ing the Redlich-Teller isotope product rule. The degenerate bending modes v (2)(E) and v(4)(F-2) were observed in the Raman spectra of NH4F and ND4F fo r the first time. The v(2) fundamental was split into doublets in the low-t emperature spectra, as predicted from a factor group analysis based on the known crystal structure, Three bands due to translational lattice modes wer e observed between 400 and 200 cm(-1) in the spectra of both isotopic molec ules. Several bands in both spectra are assigned to overtones and combinati ons involving an ammonium ion libration, but no bands directly attributable to ammonium ion librations were observed. Copyright (C) 2001 John Wiley & Sons, Ltd.